Under normal conditions skeletal muscle satellite cells (muSCs) are in a quiescent state, but when stimulated by damage, they re-enter the cell cycle to generate new fibers or self-renew to reconstitute the muSC pool. During aging muSC mediated regeneration is deeply impaired, leading to loss of skeletal muscle mass and strength (sarcopenia). Previous publications have suggested that changes in the aged muscle microenvironment lead to muSC malfunction and that a younger environment can reverse this process. Three recent publications are challenging this dogma by defining a muSC intrinsic mechanism that drives geriatric muSCs into a deep and irreversible state of senescence.
The groups lead by Pura Muñoz-Cánoves, Helen M Blau and Bradley B Olwin describe in three independent publications that muSCs from aged (20-25 months) or geriatric (28-32 months) mice are unable to repair muscle injury or replenish muSCs pool of challenged young skeletal muscle. These are the first accounts of a quiescence to senescence cellular shift being reported in skeletal muscle. This discovery is the first statement in each of the three manuscripts, but they all focus on different mechanisms, which should be connected.
Based on the gene expression profile of muSCs isolated from geriatric and progeric (SAMP8 KO) mice Muñoz-Cánoves’ group defined a sarcopenia-associated satellite cell signature in which p16INK4a (the master regulator of cellular senescence) was significantly upregulated. INK4a locus is known to be repressed by the polycomb repression complex (PRC1), of which Bim1 is an essential member. The fact that Bim1 deficient mice show premature aging and Bim1-KO muSCs show high p16INK4a expression levels indicates that PRC1 malfunction underlies p16INK4a de-repression in muSCs. The retinoblastoma (Rb) protein most likely acts downstream of p16INK4a leading to senescence. In fact high p16INK4a expression levels correlated with reduced phosphorylated Rb protein in geriatric muSCs.
The role of p16INK4 in muSC senescence is highlighted by the fact that its silencing restored geriatric and Bmi1-null satellite cell proliferation and self-renewal capacity when transplanted into mice. The authors also revealed the involvement of p16INK4a in human skeletal muscle since its expression in human geriatric muSCs prevented their myogenic functions whilst inducing senescence. In addition, genetic interference of human p16INK4a restored geriatric muSCs proliferation by reducing senescence.
After demonstrating the failure of the young environment to rescue self-renewal of aged muSCs, Blau’s group described P16Ink4a and p21Cip1 upregulation in aged muSCs. Knowing that these factors can be induced during persistent activation of stress-related pathways the authors assayed stress signaling pathways and found that aged muSCs present active p38α/β MAPK. Using aged muSCs cultures on laminin containing soft hydrogel, the authors showed that p38α/β inhibition (SB202190 or siRNA) increased the percentage of functional stem cells being able to proliferate without differentiation, suggesting that expansion of muSCs from aged mice occurs by self-renewal. Furthermore, SB202190 treatment enhanced the engraftment of muSCs from aged mice when injected into young mice. Furthermore, transplanted SB202190 treated muSCs contributed to the repair of a high proportion of recipient myofibers, and led to increases in both twitch and tetanus forces to the levels found in uninjured muscles of young mice.
After observing the poor engraftment presented by aged muSCs after transplantation into mice, Olwin’s group tested p38α/β MAPK signaling, since it acts as a molecular switch during quiescent muSCs activation. Using muSC seeding on myofiber cultures, the authors found that p38α/β MAPK phosphorylation was upregulated in muSCs. Gene expression studies on aged muSCs revealed an increase in the expression of commitment genes (MyoD, myogenin) and a reduction in the expression of genes associated with the quiescence (Pax7), asymetric division, cell growth and differentiation. A partial inhibition of p38α/β with SB203580 on aged muSCs increased the percentage of muSCs with asymetrical division and restored the number of quiescent cells nearly to the numbers seen in SCs from young mice. In order to delineate the pathway upstream of p38α/β in muSCs the authors compared FGFR1 activation and signaling in SCs from aged versus young mice. p38α/β MAPK phosphorylation was stimulated after FGF-2 treatment and reduced after inhibition of FGFR1 signaling only in young muSCs but not in aged ones, highlighting the insensitivity of p38α/β MAPK in aged muSCs to FGF addition.
In summary, three independent reports showed that aged muSCs, instead of showing their normal reversible quiescence state, switch to an irreversible pre-senescence state, which impairs their further activation. These studies revealed two pathways underlying the muSCs senescence, FGF-2->FGFR1->p38α/β and PRC1->p16INK4a->Rb. Whether these two pathways are indirectly or sequentially connected will probably be elucidated sooner rather than later. What is unquestionable is that both p16INK4a and p38α/β represent drugable targets for sarcopenia and progeria.